1. Field of the Invention
The present invention relates to a transparent conductive sheet, and more specifically it relates to a transparent conductive sheet made of a transparent plastic material, having such characteristics as excellent transparency, optical isotropy, surface smoothness, solvent resistance, interlayer adhesion and gas barrier properties, and especially low variation in gas barrier properties due to environmental humidity and low deterioration in adhesion with time, providing high display quality and reliability when used, for example, as a transparent electrode substrate for a liquid crystal display panel, and which may also be used as a transparent electrode substrate for an electroluminescence panel, electrochromic panel or the like.
2. Description of the Related Art
With the increasingly widening use in recent years of pocket-size, portable devices such as pagers, portable telephones, electronic notepads and pen-input devices, research in the field of liquid crystal display panels has resulted in the realization of liquid crystal display panels in some devices which employ plastics as substrates instead of thick, weighty and breakable glass substrates. Plastic liquid crystal substrates are lightweight, resistant to breakage, and allow more freedom of panel shape and curvature of displays.
However, the resistances to solvents including the resistance to aqueous alkali solution have not been investigated sufficiently.
When plastic substrates are used as liquid crystal display panel substrates, high performance is demanded in terms of transparency, optical isotropy, flatness, solvent resistance, interlayer adhesion and gas barrier properties.
That is, inferior transparency or optical isotropy of the substrate leads to problems such as lower brightness and contrast of the display, and coloration.
Also, low flatness of the substrate results in a non-uniform thickness of the liquid crystal layer, causing uneven orientation of the liquid crystals and thus lower display quality.
Solvent resistance is also an essential property during the step of manufacturing the liquid crystal panel, since deterioration of the appearance and flatness of the substrate is promoted as a result of low resistance to aqueous acidic solutions used for etching of the transparent conductive layer, aqueous alkali solutions used for the resist development, and organic solvents such as N-methylpyrrolidone and .gamma.-butyrolactone used for formation of the liquid crystal aligning film.
Furthermore, low adhesion between the layers laminated on the substrate leads to peeling and cracking during production of the panel, and as the adhesion with time deteriorates, the long-term reliability of the panel is lowered.
Finally, when the gas barrier property of the substrate is poor, oxygen, nitrogen and water vapor seep in and out of the liquid crystal layer through the substrate, causing deterioration of the liquid crystals and generating air bubbles in the layer. For this reason, it is believed preferable for the gas barrier properties of substrates to be sufficiently high to provide a maximum oxygen permeability of 10 cc/m.sup.2 .multidot.day.multidot.atm under usual conditions, i.e., measuring conditions of 30.degree. C., 50% RH, and a maximum water vapor permeability of 20 g/m.sup.2 .multidot.day.multidot.atm under usual conditions, i.e., measuring conditions of 40.degree. C., 90% RH. However, it is preferable under severe conditions, for example, for use in a vehicle, to have a maximum oxygen permeability of 10 cm/m.sup.2 .multidot.day.multidot.atm at 30.degree. C. and 90% RH and a maximum water vapor permeability of 10 g/m.sub.2 .multidot.day.multidot.atm at 40.degree. C. and 90% RH.
In contrast, the oxygen permeability of a 100 .mu.m-thick polycarbonate sheet, for example, is high at about 1200 cc/m.sup.2 .multidot.day.multidot.atm under measuring conditions of 30.degree. C., 90% RH, and the water vapor permeability is also high at about 50 g/m.sup.2 .multidot.day.multidot.atm under measuring conditions of 40.degree. C., 90% RH.
A plastic substrate is described in Japanese Unexamined Patent Publication No. 56-130010, and a plastic substrate developed for improved gas barrier properties is described in Japanese Unexamined Patent Publication No. 61-41122, No. 61-73924, No. 3-9323, etc.
As gas barrier layers for providing gas barrier properties, there have been proposed and employed layers made of organic-based materials such as polyvinyl alcohol-based resins, polyacrylonitrile, polyvinylidene chloride, etc. and layers of metal oxides such as silicon oxide.
Thus, when a plastic sheet is used as the substrate, a layer with an excellent gas barrier property must be provided. Such gas barrier layers which have been proposed include the aforementioned organic-based materials such as polyvinyl alcohol-based resins, polyacrylonitrile, polyvinylidene chloride, etc. and metal oxide layers made of silicon oxide or the like.
Gas barrier layers made of metal oxides, representative of which is silicon oxide, have gas barrier properties which are virtually unaffected by environmental humidity; however, because of their low scratch resistance and alkali resistance, they require the lamination of a layer with solvent resistance (hereunder referred to as "solvent-resistant layer"). To improve scratch resistance, the patent mentioned above has proposed lamination of a metal oxide layer and a cured resin layer, but not all the cured resin layer has a sufficient solvent resistance. Further, upon testing lamination of thermosetting epoxy resins and ultraviolet-curing acrylic resins, which have solvent resistance, on metal oxide layers, it was found that the adhesion was generally low and become insufficient, after durability testing.
On the other hand, polyvinyl alcohol-based resins such as polyvinyl alcohol or an olefin-vinyl alcohol copolymer are used for the gas barrier layer since they have a gas barrier property more excellent than a metal oxide layer under usual conditions of 50% RH or less.
However, plastic sheets used for applications which generally require optical isotropy, such as liquid crystal display panels, for example polyvinyl alcohol-based resins have poor adhesion with polycarbonate and carbonate copolymer sheets, and thus direct coating on such plastic sheets results in insufficient adhesion and proneness to peeling.
As a measure to solve these problems, the means described in Japanese Unexamined Patent Publication No. 63-71829 and No. 3-9323 has been applied, i.e. an anchor coat layer with good adhesion is provided on the plastic sheet, and then a polyvinyl alcohol-based resin layer is laminated over it.
The use of polyurethane resins, aqueous polyesters and the like as the aforementioned anchor coat layer is known. The anchor coat layer, however, is usually formed by a wet coating process, and since the compositions used for the above-mentioned known anchor coat layers are not quick-drying, the coating layer surface is sticky and cannot be quickly made into a tack-free, or non-adhesive state.
Consequently, when the anchor coat layer is further coated with a polyvinyl alcohol-based resin layer by a wet coating process, the coating surface cannot be prepared uniformly, which results in the problem of an uneven coating, and poor optical properties such as greater haze due to the rough interface. This problem is caused because the tacky surface is soft and prone to be roughen during the coating step.
In addition, because of this tackiness the film which has been laminated with the anchor coat layer also sticks to itself when it is wound into a roll, and such handling becomes impossible, making it more difficult to work with the film during the production process.
The present invention has been accomplished in light of such circumstances, and its object is that of providing a wide variety of transparent conductive sheets prepared with excellent transparency, optical isotropy, surface smoothness, solvent resistance and gas barrier properties, and particularly excellent adhesion between layers and durability, by laminating an intermediate layer with excellent adhesion and further a solvent-resistant layer with excellent durability and solvent resistance on a gas barrier layer of a metal oxide layer or a polyvinyl alcohol-based resin layer, and solvent resistance layer.